Conventionally, railroad vehicles such as a Shinkansen bullet train are required to travel at a higher speed and to have a large transportation capacity. In order to satisfy the requirements, it is necessary to reduce the size and the weight of each ancillary device mounted in a railroad vehicle, whereas the vehicle-mounted transformer is increased in capacity in order to satisfy the requirements for high-speed traveling and mass transportation.
In recent years, there are increasing demands for a low-floor vehicle for the purpose of achieving a barrier-free design. Accordingly, with regard to the devices disposed under the floor of the vehicle such as a vehicle-mounted transformer, there is a strong demand for reduction in size and weight.
There is also a railroad vehicle that runs through a plurality of sections in which AC voltages supplied from an overhead wire and the like are different. In the transformer mounted in such a vehicle, even when the AC voltage supplied to the vehicle varies, the transformer is required to output a desired AC voltage.
Japanese Patent Laying-Open No. 63-209113 (PTL 1) discloses a tap changer for changing the turns ratio in the transformer, which is configured for suppressing the oscillating voltage generated by a surge voltage.